Two-disease locus model: sib pair method using information on both HLA and Gm.

An adaptation of the sib pair method is given for testing the independence of transmission of Gm and the disease, using variable X (which gives information on the phenotypic identity of a sib pair for Gm). Then the joint distribution of IBD for HLA and X for Gm among affected sib pairs was derived under different two-locus models (multiplicative and other) in which one locus is strictly linked to HLA and the other to Gm. We also propose a test for a joint effect of HLA and Gm and whether this effect is multiplicative or not.